An important requirement for high quality magnetic resonance (MR) applications such as nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) is to have accurate information about the spatio-temporal electromagnetic field evolution during the various measurements.
It has long been recognized that the spatiotemporal low-frequency magnetic field evolution in a volume of interest can be determined by means of an array comprising a plurality of magnetic field probes arranged within or in the vicinity of the volume of interest (Barmet et al. ISMRM 2009 p. 780). Such arrangements are occasionally referred to as “dynamic magnetic field cameras”. It allows for observing the spatiotemporal low-frequency magnetic field evolution with the full bandwidth of the latter.
A promising approach for magnetic field cameras relies on arrays of NMR probes, i.e. on probes exploiting the magnetic field dependence of NMR signals (DeZanche et al. MRM 60:176-186 (2008); Sipilä et al. ISMRM 2010 p. 781). In particular, such arrangements use probes operating in a pulsed NMR mode.
Moreover, as already disclosed in EP 1 582 886 A1, arrays of NMR probes can be used as a monitoring setup during execution of an NMR or MRI sequence. The magnetic field information thus obtained is used for an improved reconstruction of the images or spectra and/or for adjusting the MR sequence so as to account for imperfections in the magnetic field behavior.
It has been proposed to use frequency-division multiplexing for concurrent imaging and field monitoring, e.g. with four field probes (Pavan et al. ISMRM-ESMRMB Joint Annual Meeting Proceedings (2010) XP-002658386). In this method, the signal of each monitoring probe is combined with that of one imaging coil so as to jointly use only a single receiver channel, enabling concurrent field monitoring without the need for additional spectrometer hardware. When using NMR field probes based on 19F together with an 1H imaging set-up, the fluorine signal of each field probe was mixed up to a frequency close to the Larmor frequency of the protons, although not overlapping with the same. In order to reduce signal-to-noise degradation caused by the superposition of noise from the shifted-fluorine and from the proton channels, it was suggested to use band-pass filtering before combining the two signal channels. In particular, the magnetic field probe channel would be provided with a bandpass filter that selectively transmits the up-converted fluorine signal.
A shortcoming of the magnetic field camera and monitoring arrangements and methods used so far is caused by the fact that in order to avoid unacceptable interference with the principal measurement and/or damage of certain components the field probe data acquisition is limited to certain time windows of the principal measurement, where no radio-frequency (RF) excitation pulse is played out. Therefore, the information supplied by the NMR field probes actually does not cover an important time region of the principal measurement. An additional drawback is the fact that in general the sequence of pulses used to excite the NMR field probes is generally not synchronized to the pulse sequence of the principal measurement; consequently the field probe excitation pulses might occur at any time during the actual MR sequence.
Accordingly, it is an object of the present invention to provide an improved dynamic field camera arrangement and methods for operating the same.